High frequency coaxial cable

Abstract

A high frequency coaxial cable (1) contains a concentric internal conductor (3), a cylindrical insulating layer (7) surrounding the internal conductor (3) and made of a foamed plastic containing more than 50% by volume of air, a tubular external conductor (9) placed over the insulating layer (7), and an outer sheath (11). Cutouts (15, 15') are made in the insulating layer (7) by removing insulation material starting at the periphery (13) of the insulating layer and extending radially inward. These cutouts (15, 15') make it possible to increase the dielectric properties of the insulating layer between internal conductor (3) and external conductor (9).

What is claimed is: 
     
       1. A high frequency coaxial cable comprising: (a) an internal conductor;   (b) a cylindrical insulating layer concentrically surrounding the internal conductor and being made of a foamed plastic containing more than 50% by volume of air, the cylindrical insulating layer having cutouts defined therein that extend radially inward starting at a periphery of the cylindrical insulating layer, the cutouts have a width in a range of between 10 and 50 mm and are formed by removing a portion of the foamed plastic from the cylindrical insulating layer;   (c) a tubular external conductor placed over the insulating layer; and   (d) an outer sheath surrounding the tubular external conductor.   
     
     
       2. A high frequency cable as claimed in claim 1, wherein the cutouts extend radially inward to the internal conductor. 
     
     
       3. A high frequency cable as claimed in claim 1, further comprising an intermediate layer placed between the internal conductor and the insulating layer. 
     
     
       4. A high frequency cable as claimed in claim 3, wherein the cutouts extend radially inward to the intermediate layer. 
     
     
       5. A high frequency cable as claimed in claim 1, wherein the insulating layer contains cylindrical sections in which no cutouts have been made therein. 
     
     
       6. A high frequency cable as claimed in claim 5, wherein the cylindrical sections are spaced at least 2.0 meters apart. 
     
     
       7. A high frequency cable as claimed in claim 6, wherein the cylindrical sections are spaced apart by varying distances along the length of the cable. 
     
     
       8. A high frequency cable as claimed in claim 5, wherein the cylindrical sections are spaced apart by varying distances along the length of the cable. 
     
     
       9. A high frequency cable as claimed in claim 1, wherein the cutouts have a helical shape. 
     
     
       10. A high frequency cable as claimed in claim 9, wherein the helically shaped cutouts are of different pitch and overlap each other. 
     
     
       11. A high frequency cable as claimed in claim 9, wherein the helical cutouts have a pitch that does not exceed 7.5 cm. 
     
     
       12. A high frequency cable as claimed in claim 9, wherein the helical cutouts have a pitch that varies along the length of the cable. 
     
     
       13. A high frequency cable as claimed in claim 1, wherein the cutouts have a circular shape. 
     
     
       14. A high frequency cable as claimed in claim 13, wherein the circular shaped cutouts are spaced apart a distance not exceeding 7.5 cm. 
     
     
       15. A high frequency cable as claimed in claim 13, wherein the circular shaped cutouts are spaced apart distances that vary along the length of the cable. 
     
     
       16. A high frequency cable as claimed in claim 1, wherein the cutouts have a substantially semi-circular shape. 
     
     
       17. A high frequency cable as claimed in claim 16, wherein the semi-circular shaped cutouts are spaced apart a distance not exceeding 7.5 cm. 
     
     
       18. A high frequency cable as claimed in claim 16, wherein the semi-circular shaped cutouts are spaced apart distances that vary along the length of the cable. 
     
     
       19. A high frequency cable as claimed in claim 1, wherein the cutouts extend parallel to the longitudinal axis of the high frequency cable. 
     
     
       20. A high frequency cable as claimed in claim 1, wherein the foamed plastic contains more than 70% by volume of air. 
     
     
       21. A high frequency cable as claimed in claim 1, wherein the foamed plastic of the insulating layer is foamed polyethylene. 
     
     
       22. A high frequency cable as claimed in claim 9, wherein the helical cutouts have a pitch which is smaller than: ##EQU3## where f=frequency of reflection factor peak in MHz; v r  =relative propagation speed in %; and l=the pitch of the helical cutouts in meters.   
     
     
       23. A high frequency cable as claimed in claim 13, wherein there is a space between adjacent ones of said circular cutouts which is smaller than: ##EQU4## where f=frequency of reflection factor peak in MHz; v r  =relative propagation speed in %; and l=the pitch of the helical cutouts in meters.   
     
     
       24. A high frequency cable as claimed in claim 1, wherein the cutouts are formed after the foamed plastic is hardened.